1. Field of the Invention
The present invention is directed to an x-ray diagnostic device of the type having an x-ray source with a two-dimensional, i.e. planar, detector arranged opposite thereto for x-rays emitted by the x-ray source as a conical, e.g. spherical or pyramid-shaped, x-ray beam, and with a positioning device for an examination subject arranged between the x-ray source and the detector, wherein the x-ray source can be rotated in tandem with the detector around a system axis and a computer reconstructs images of the examination subject from the output signals of the detector thereby obtained.
2. Description of the Prior Art
X-ray diagnostic devices are known, wherein the x-ray source and the detector are mounted opposite each other at an arc-shaped support. Since the x-ray source and the detector thus are not mounted at a closed ring, differing from conventional computed tomography, such x-ray diagnostic devices are particularly suited for intra-operative application because a patient located on the positioning device is easily accessible due to the arc-shaped support.
The output data of the detector that are acquired due to the arc-shaped structure of the support given only a partial (incomplete) circuit of the x-ray source and detector around the system axis, are reconstructed to produce images of slices of the examination subject using a 3D-back projection algorithm that is related to the Feldkamp-algorithm, as is for the conventional computed tomography (see L. A. Feldkamp, L. C. Davis, J. W. Kress, xe2x80x9cPractical Cone Beam Algorithmxe2x80x9d, J. Opt, Soc. Am., Vol. A6, pp. 612-619, 1984). This reconstruction method is suitable for a small angle of beam spread of the x-ray beam and thereby of the detector system. Image artefact must be expected, however, for slices of the examination subject to be reconstructed, that lie far outside of the center plane, i.e. that plane which is at a right angle to the system axis that contains the focus of the x-ray source. While these artefacts are not detectable in the case of high contrast applications (employing x-ray devices of the initially described type as described in H. Barfuxcex2, Digital 3D-Angiography, VDE-Technical Report Volume 54: The Digital Hospital, VDE-Publishing, 1998), they cannot be ignored for low contrast applications. This permits images only of a limited area of the examination subject to be reconstructed free of image artefacts at least for low contrast applications with x-ray diagnostic devices of the initially described type.
In computed tomography, this problem can be addressed by spiral scanning of the examination subject, wherein the x-ray source and the detector, and the positioning device with the examination subject lying thereon, are shifted linearly relative to one another with continuous rotation of the x-ray source and detector around the system axis. Such a procedure is ruled out, however, in the case of x-ray diagnostic devices of the initially described type having an arc-shaped support for the x-ray source and the detector, because a continuous rotation of x-ray source and detector around the system axis is precluded in the case of such a structure.
An object of the present invention is to provide an x-ray diagnostic device of the type initially described wherein larger volume areas of an examination subject can be scanned without continuous rotation of x-ray source and detector around the system axis.
This object is inventively achieved in an x-ray diagnostic device having an x-ray source with a two dimensional detector arranged opposite thereto for x-rays emitted by the x-ray source as a conical x-ray beam, and with a positioning device for an examination subject disposed between the x-ray source and the detector, wherein the x-ray source can be rotated in an oscillating fashion in tandem with the detector around a system axis, and at the same time, the x-ray source and the detector and the positioning device are linearly mounted relative to one another substantially in the direction of the system axis, and when a computer reconstructs images of the examination subject from the output signals of the detector obtained by such scanning.
In contrast to conventional computed tomography, an oscillating rotation of the x-ray source and the detector around the system axis takes place in the case of the inventive x-ray diagnostic device, while, at the same time, the x-ray source and the detector and the positioning device on the other are moved relative to one another in the direction of the system axis. In this manner, it is possible to also scan larger volumes of an examination subject without continuous rotation of the x-ray source and detector around the system axis. Due to the two dimensional detector and the spherical or pyramid-shaped configuration of the x-ray beam, the examination subject is completely scanned in the direction of the system axis given adequate amplitude of the oscillating rotation and given a speed of the relative movement between x-ray source and detector and the positioning device which is not too fast, the limitation associated with x-ray diagnostic devices of the initially described type of being to produce images free of image faults only in a limited volume is eliminated.
In a preferred embodiment of the invention, the x-ray source and the detector are mounted opposite one another at an arc-shaped support. In the case of the support being a C-arm, its center axis substantially coincides with the system axis and the C-arm is moved back and forth around its center axis to generate the joint oscillating rotation of the x-ray source and detector. As experience from x-ray angiography shows, a C-arm offers medical personnel good access to a patient to be examined and, if necessary, treated.
In order to enable a complete scanning of the examination subject, the oscillating rotation ensues in one version of the invention with an amplitude that is at least equal to 180xc2x0 plus the angle of the beam spread, with the angle of the beam spread emitted from a focus of the x-ray source being measured in the center plane.
In order to assure a complete scanning of the examination object in the direction of the system axis, in a further version of the invention the x-ray source and the detector, and the positioning device, are shifted relative to one another in the direction of the system axis by an amount (distance) per period of the oscillating movement, that is at most equal to xcex94Zmax, with       Δ    ⁢          xe2x80x83        ⁢          z      max        =            Δ      det        ·                            R          f                -                  Δ                      Obj            /            2                                                R          f                +                  R          d                    
wherein
xcex94det: width of the detector measured in the direction of the system axis
xcex94obj: span of the area to be scanned of the examination subject measured transverse to the direction of the system axis
Rf: distance of the focus of the x-ray source from the system axis, and
Rd: distance of the detector from the system axis.
In a further embodiment of the invention, the computer calculates individual image points on the basis of those x-rays passing through the respective image points at different projection angles in the reconstruction of images of the examination subject, which have the smallest gradient relative to the center plane.
In a preferred embodiment of the invention, the computer calculates individual image points for the reconstruction of images of the examination subject on the basis of x-rays passing through the respective image point at different projection angles, with several x-rays exhibiting different gradients relative to the center plane being taken into consideration in a weighted average per projection angle, with the result that the x-ray dose applied to the examination subject being better utilized.